Question 2 {10 marks} The Kronig-Penney model describes the electronic behaviour of a free electron in solid. {a} State the assumption of electron energy in this model. [4 marks] (b) The energy band of the Kronig-Penny model for a crystal is shown in the gure below. Figure 4 Sketch the rst and second derivatives of the energy band { E Fem\" k and not a\" 1L 1 rem-rm It) (t ' [6 marks]